A liquid crystal display (LCD) device, a representative example of a display device, is used as a display device of almost every electronic device such as a display of a television, a monitor, or the like, a portable device such as a mobile phone, a portable multimedia player (PMP), an MP3 player, a digital camera, or the like, a public information display (PID) for displaying information in a public area, an indoor/outdoor advertisement display, or the like.
A general LCD device according to a conventional technique includes a plurality of pixels arranged in a two-dimensional (2D) matrix form. Here, each pixel includes transparent substrates facing each other, transparent electrodes formed on the substrates, and a liquid crystal encapsulated between the transparent electrodes. In case of a color display, color filters constituting sub-pixels are formed on the surfaces of the transparent substrates in order to express colors such as red, green, blue, and the like. Polarizer plates (or polarizer layers) which are perpendicular to each other and have polarization characteristics are disposed on both sides of the transparent substrates. At usual times, the polarizer plates block light irradiated from a backlight unit. However, when a voltage is applied to the liquid crystal, an alignment of the liquid crystal is rotated to change a phase of light and the polarizer plates allow light irradiated from the backlight unit to be transmitted. Also, in order to independently drive liquid crystal of the sub-pixel region, generally a thin film transistor (TFT) driving circuit is integrated in the form of a 2D matrix form on the lower transparent substrate constituting a liquid crystal panel.
In the LCD device, pixels constituting an image do not emit light by themselves. That is, the LCD device displays an image by intermitting illumination light irradiated from a back light source by combining two or more polarizer plates. In this case, a half or more of back light is lost by the polarizer plates and only about one-third of light which has passed through the polarizer plates after passing through spatially separated color filters is used for displaying an image, and since a light loss occurs due to other optical film factors, so in an optical aspect, an LCD device has very low light usage efficiency.
Meanwhile, demand for a display device which may be able to provide excellent picture quality even in an ambient light environment such as strong natural light, or the like, in a field related to a portable device such as a portable phone, a tablet computer, a personal digital assistant (PDA), a portable multimedia device, or the like, a public information display, and outdoor advertisement display device, or the like, is on the rise. However, the existing transmissive LCD device has a problem in which brightness of display finally output through liquid crystal and a plurality of polarizer plates after being irradiated from a backlight is relatively low in comparison to ambient brightness by natural light. Also, readability and visibility are very low due to an influence of ambient light reflected from a surface of a liquid crystal display. Also, even in the case of a TV, a computer monitor, or the like, employing an LCD device, visibility is degraded due to an environment such as intensive indoor illumination. There has been a method of increasing brightness of a back light source based on ambient light condition after detecting brightness of ambient light. However, there is a limitation in increasing brightness of a back light source due to efficiency of a light emitting element and a technical problem such as heating, or the like, and in this case, power consumption is also rapidly increased.
Meanwhile, in order to secure visibility even in a bright ambient light environment and reduce power consumption by a backlight unit, a reflective LCD device using bright ambient light such as natural light, or the like, as a light source has been developed. Like a general transmissive LCD device, a reflective LCD device includes a liquid crystal layer, a color filter, a polarization filter or a polarizer plate, a transparent substrate, and the like, and an operation principle for displaying an image is similar to that of a transmissive LCD device. However, in a reflective LCD device, color filter of a certain region is removed a reflective layer, or the like, is added. The reflective LCD device is different from the transmissive LCD device, in that light made incident from the outside through the region from which a color filter was removed is reflected through the reflective layer to adjust brightness through liquid crystal of each sub-pixel and allowed to pass through a color filter to display an image. Since the reflective LCD device does not require an additional light source, power consumption is low and readability can be improved even in a bright ambient light environment. However, the reflective LCD device has shortcomings in that a picture quality may be degraded in an indoor or dark environment.
Recently, a trans-reflective LCD device, or the like, has been studied and developed as an LCD device combining a reflective LCD device using ambient light and a transmissive LCD device using an existing back light illumination. However, in the trans-reflective LCD device, a region for a reflection operation and a region for a transmissive mode operation are areally divided and divided respective pixels of the display device are combined to be configured, so that a degradation of spatial resolution in case of transmissive mode driving and reflective mode driving cannot be avoided and a maximum light brightness cannot also be achieved in each mode. In particular, when the fact that portable devices such as portable phones, or the like, are used without being limited to places including indoor and outdoor areas and the fact that demand for low power consumption due to battery driving is increasing, and the like, are considered, a substitute for overcoming the limitation of the existing LCD devices is required.